1. Field of the Invention
The present invention relates to the separation of isoolefins from streams containing mixtures of an isoolefin and the corresponding normal olefin. The present invention is especially useful for the separation of isobutene from streams containing n-butenes.
2. Prior Art
Isoolefins of four carbon atoms are difficult to separate from the corresponding normal olefin by simple fractionation because of the closeness of their boiling points. In prior art processes as generally practiced commercially, the isoolefin is selectively absorbed by sulfuric acid and the resulting isoolefin-containing sulfuric acid extract is then diluted and heated or treated with steam to separate the isoolefin.
Isobutene and diisobutene are of significant value, having diverse applications. For example, isobutene is one of the comonomers for butyl rubber and diisobutene is an intermediate in the preparation of detergents. The isobutene oligomers are useful as polymer gasoline. The n-butenes are required in pure form for homopolymerization and as feeds for the oxidative production of butadiene. One manner of separating these components is to pass the mixture through a cold acid extraction procedure wherein the stream is fed into a bath of concentrated sulfuric acid. Separation is achieved by virtue of the solubility of the isobutene in the sulfuric acid, the n-butenes and other hydrocarbons present passing overhead, for example as shown in U.S. Pat. Nos. 3,546,317 and 3,823,198.
Other processes have used various catalysts for converting the isobutene to diisobutene which is then easily separated from the product stream. For example, a process using a molecular sieve and elevated temperatures is disclosed in U.S. Pat. No. 3,531,539; U.S. Pat. No. 3,518,323 employs a supported nickel oxide catalyst; and U.S. Pat. No. 3,832,418 employs a Group VI or VIII metal deposited on acidic, amorphous silica-alumina in the same manner.
More recently, U.S. Pat. No. 4,215,011 disclosed the use of acid cation exchange resin in a heterogenous combination reaction-distillation system for the selective dimerization of isobutene in the presence of normal butenes.
Although the present process is suited to treat other isoolefin-normal olefins mixtures, it is of particular significance for the recovery of product streams with sufficiently low levels of isobutene to be processable to obtain useable n-butenes and particularly butene-1 which is the n-butene isomer employed in homopolymerization to produce polybutene or copolymerization with other monomers and as the preferred feed for oxidative dehydrogenation to produce butadiene-1,3.
It is a principal feature of the present process that the amount of isobutene in the stream is reduced to levels sufficiently low to allow further separation of a useful butene-1 product. It is another feature of the present process that a very useful product is produced from the isobutene, i.e., polymer gasoline. It is a particular advantage of the present process that it may be operated to obtain the above results with a limited loss of butene-1.
It is a further advantage of the present process that the diisobutylene product produced according to the present process may have lower ratios of codimer (the reaction product of n-butenes and isobutene) and triisobutene to diisobutene than the prior cold acid method of removing isobutene from C.sub.4 streams.
Another feature of the present process is the substantial energy saving over the cold acid method of isobutene removal and a deduction in capital expenditures to replace and/or repair processing equipment that has failed due to the corrosive nature of the sulfuric acid.